Current purge systems for engines having a turbocharger unit use a venturi vacuum generator (i.e., a vacuum pump) to allow purge of the evaporative system while the turbocharger unit is activated (manifold under positive pressure). To create this vacuum pump, significant amounts of the air flow from the turbocharger unit are used, thereby reducing the power increase to the engine created by the turbocharger unit. One attempt to limit the amount of pressurized air running through the vacuum pump, and temporarily maximize power, is to include a turbo bypass switching valve (BSV) to alter the amount of pressurized air going to the vacuum pump. The BSV is opened and closed to control how much air is diverted from the turbocharger unit when the turbocharger unit is activated. There is a mass airflow sensor downstream of the turbocharger unit which monitors the airflow during naturally aspirated conditions, and conditions when the turbocharger unit is activated and generating pressurized air.
Because the mass airflow sensor is located in a separate flow path from the vacuum pump, the vacuum pump in effect creates a secondary flow path from the airbox to the manifold, circumventing the mass airflow sensor, and has the potential to allow unmonitored air into the engine during naturally aspirated conditions. In order to block this secondary flow path, turbo purge systems have been developed with an additional check valve between the BSV and the turbo outlet, which allows flow in conditions when the turbocharger unit is generating pressurized air to drive the venturi vacuum generator, but blocks flow under naturally aspirated (vacuum) conditions.
However, incorporating the additional valve into this secondary flow path has been met with increased difficulty and expense due to packaging restrictions within the vehicle, and the need to prevent the valve from improperly altering air flow. Accordingly, there exists a need for a valve assembly which allows air to pass to the venturi vacuum generator when the turbocharger unit is activated, and closes under naturally aspirated (vacuum) conditions, without altering air flow.